1. Field of the Invention
The present invention relates to a shortwave semiconductor laser using a new Group III-V compound semiconductor material.
2. Description of the Related Art
As a consequence of the development of high-speed and high-density information processing systems, demand has arisen for the realization of a shortwave green-light semiconductor laser (LD).
Group III-V compound semiconductor materials having a wide band gap considered to be most suitable for realizing a green semiconductor laser include nitrides and phosphides of light Group III elements such as BN (4 or 8 eV), AlN (6 eV), GaN (3.4 eV), InP (2.4 eV), AlP (2.5 eV), and GaP (2.3 and 2.8 eV). However, it is difficult to synthesize a high pressure phase (c-BN) having an sp3 bonding by using BN although BN has a sufficiently wide band gap. Furthermore, it is difficult to perform impurity doping by using BN. In addition, BN has three different polymorphisms and tends to form a mixture. For the above reasons, therefore, BN is not suitable. InN has an insufficiently wide band gap for the realization of a green laser and poor thermal stability. In addition, only a polycrystal can be normally obtained by using InN. The band gap of each of AlP and GaP is slightly insufficient. AlN and GaN both have sufficiently wide band gap and good stability and therefore are suitable for shortwave light emission. However, the crystal structure of AlN and GaN is of a Wurzeite type (to be referred to as a WZ type hereinafter) and has high ionicity. Because of this, lattice defects easily occur, and thus a low-resistance p-type semiconductor cannot be obtained.
With the aim of overcoming the above drawbacks, attempts have been made to obtain a material having a wide band gap by mixing B and N with a Group III-V compound semiconductor which does not contain either of these materials. However, the lattice constant of a conventional III-V compound semiconductor material differs from that of the material containing B and N by as much as 20% to 40% and both materials also have different crystal structures. As a result, a stable crystal cannot be obtained. For example, in order to mix N in GaP, the mixing ratio of N to GaP was 1% or less. Consequently, a sufficiently wide band gap cannot be obtained.
According to studies carried out by the present inventors, a low-resistance p-type crystal cannot be obtained by use of GaN or AlN essentially because due to their high ionicity, a defect can easily occur and because the crystal structure of GaN and AlN is not a Zinc Blende type (to be referred to as a ZB type hereinafter) but rather a WZ structure.
As described above, no conventional semiconductor material satisfies the requirements for realizing a green semiconductor laser, i.e., a wide band gap of, for example, 2.7 eV, p and n conductivity type control, and good crystal quality. Although a nitride such as AlN or GaN is a suitable material for obtaining a wide band gap, a low-resistance p-type layer cannot be obtained therefrom.